The invention concerns a tire with a radial carcass reinforcement intended to equip a heavy vehicle such as a transport vehicle or construction machinery. This is in particular a tire which has an axial width greater than 37 inches.
Such a tire, intended generally to bear heavy loads, comprises a radial carcass reinforcement, and a crown reinforcement composed of at least two working crown plies, formed by non-extensible reinforcement elements, crossed from one ply to the next and forming, with the circumferential direction, equal or unequal angles, lying between 10° and 45°.
The crown reinforcements of radial tires, and more particularly with regard to very large tires, are subjected to great deformation, generating between the edges of two cross-plies longitudinal and transverse shear stresses (the longitudinal shearing is greater than the transverse when the cables of crossed plies form small angles with the circumferential direction), at the same time as a delamination stress, a radial stress having a tendency to radially separate the edges of two plies. The said stresses are due firstly to the inflation pressure of the tire, which means that the so-called belting pressure between carcass reinforcement and crown reinforcement tends to cause the circumferential expansion of the said crown reinforcement. The said stresses are also due to the load carried by the tire in running with the arising of a contact surface between ground and tire. The said stresses are also due to the drifting of the tire during running. These stresses generate cracks in the rubber mix adjacent to the end of the shortest ply, cracks which propagate in the said mix and which are detrimental to the fatigue strength of a crown reinforcement and therefore of the tire.
An appreciable improvement to the fatigue strength has been procured by the use in the crown reinforcement of at least one protective crown ply having an axial width greater than the width of the axially widest working ply.
“Axial” means a direction parallel to the rotation axis of the tire and “radial” means a direction intersecting the rotation axis of the tire and perpendicular thereto. The rotation axis of the tire is the axis about which it turns in normal use. The circumferential mid-plane is a plane perpendicular to the rotation axis of the tire which divides the tire into two halves. A radial plane is a plane which contains the rotation axis of the tire.
Another solution as described in the patent FR 2 421 742 consists of more favorably distributing the stresses generating separation between working crown plies, resulting from the drift of the tire, by multiplying the number of working plies.
Increasing the number of working plies is not without drawback, in particular at the centre of the crown reinforcement where the number of plies has a direct influence on the flexing rigidity of the tire crown. When this rigidity increases, the result is that impacts occurring on the tire crown, such as in particular passing over large pebbles, may result in irremediable damage to the tire, because of this increased rigidity.
Patent application WO 00/54992 also proposed, in order to avoid this drawback, producing a working crown reinforcement consisting of at least three continuous working plies and at least one half-ply, on each side of the circumferential mid-plane, disposed between the edges of at least two radially adjacent continuous working plies, whose particularity is in particular to present an angle to the circumferential direction greater than 25° and greater by a quantity of between 5° and 15° than the largest of the angles of the continuous working plies. The results obtained with this type of architecture were entirely satisfactory for the tire dimensions tested.
In their studies and in particular during the study of the production of tires of larger size, in particular where the axial width is greater than 50 inches, the inventors set themselves the task of defining a tire crown architecture for heavy machinery making it possible to obtain satisfactory fatigue strength, in particular by improving the circumferential and transverse rigidities in order to limit the shear stresses and preserving flexibility of the crown.